As soon as you bind an item to a character (by equipping, placing a bound mod into, or consuming/using it), it will be unlocked in Collections for that character. That character can then redeem infinite copies of that item for free.
If you log onto another character on the same account, you can see all those items and choose to pay an additional fee to unlock an item account-wide (which also gives infinite free redemptions). There's no way to just do the unlock for a single character via Collections; if you wanted to do that, you'd need to obtain a second unbound copy of the item and bind/use it on the desired character.
Note that some items cannot be redeemed from Collections even though they appear there. Off the top of my head:
- Any unique items from Cartel Pack Reputation vendors
- Individual purple chest pieces containing mods
- Mod packs for your starship
Also, I should clarify that you need to bind an entire gear set on a single character for it to be unlocked in Collections. Everything else is single-item. After the Collections unlock, which you should see a mention in Chat about, it is safe to discard the unlocked items thanks to the infinite free redemptions (as long as they do not fall into the exceptions above).