I've seen articles and videos saying a particular hero is strong on this lane (eg. Mid) and weak on another lane. Where do they base this from?
Note: Some in-game terminologies have been used in this answer. If you are new to dota 2 then please check out Fodder's answer first.
What this means is that a certain hero's skills synchronize well with him/her being played in that certain lane or it depends on their needs (farm - wise). Also in pro games their entire draft depends on the meta (usually).
For example, mid heroes can be of two types. A farming mid like invoker, dragon knight, tinker etc. or a gank oriented mid like Queen of pain, pudge etc.
Most heroes are played only in mid because they need the fast exp. growth.
Another important point is the farm position. What this means is which hero will need the most farm to be most effective.
Position 1: This is your safe lane carry.
Position 2: Mid Laner.
Position 3: Offlaner.
Position 4: Roaming Support.
Position 5: Another support.
Technically all heroes can be played at any position but most heroes usually have a fixed role in each meta.
The two most common ways of laning are, 2-1-2 and 1-1-3.
- 2-1-2 Two people in offlane, one in mid and another 2 in safelane. You will see this kind of laning in pub matches especially in the lower tier. Here, position 1 and 5 are in safelane, position 2 in mid and position 3,4 in offlane.
- 1-1-3 One offlane, one mid, and 3 in safe lane. Position 3 offlane, pos 2 mid and the rest in safelane. This is called having a trilane. You might also sometimes encounter an aggressive tri-lane which is basically 3 people in offlane.
Heroes like anti mage and spectre who need a lot of farm to be effective will be tough to play offlane or even mid. Also trying to carry with a support would be very difficult too.
Note: If you need more info just say so I'll add more.
Some heroes have an advantage getting runes (not as much of an issue with 5 runes and sanc with in patch 7.00). E.g. Queen of Pain, Shadow Fiend and Dragon Knight can clear waves pretty quickly with spells, and the rune, combined with a bottle, gives them mana / HP regen every 2 minutes.
Some heroes also have a better time farming with the hill. E.g. Templar Assassin is quite short ranged, especially early in the game, but the hill makes it harder for ranged heroes to harrass her out of the lane.
And as @Auditore mentioned, the mid lane generally tends to be a solo lane, so heroes that get a power spike once they reach level 6 and get their ultimate (e.g. Queen of Pain, Rubick, Pudge), or heroes that just need levels in general (e.g. Invoker).
The creeps tend to naturally meet closer to the tower (hence the name "safe" lane), so it means heroes can generally be a bit riskier when farming because it's easy to retreat back to the tower if something goes wrong. Allied heroes can also teleport to the tower for a rescue operation, if needed. This means that heroes that are weaker in the earlier stages of the game can be easily protected so that they can farm up.
The lane is also quite close to the jungle, so if the lane gets pushed out, the support can pull a wave of creeps into the jungle so that the next wave meets closer to the tower again, or the carry can pop into the jungle for a bit - which is great for heroes like Anti-Mage once he has his battlefury, as he can farm the lane, then blink around farming the jungle.
Heroes that shine in the offlane tend to be heroes who can farm well, despite the disadvantages of the offlane, or heroes that can play aggressively and try to shut down the opposing safe lane farmer. Offlane heroes might also need to be heroes who have an escape of some sort, or are tanky, as they are often easy ganking targets due to the fact that they will generally be far from the tower, and in some cases, a solo laner.
Dark seer is good example, as he can still farm somewhat safely with ion shell (put on creeps), he has an escape in his surge spell, and ion shell can disrupt a melee carry's farm.