The in-game editor is a lighter version than the main editor.
You can edit a players or staff members attributes, you can move a player from your club to another and vice versa, (eg. go on Messi's profile and select the editor tool. You'll see "Move to your club" or "Move to another club") You can change a whole teams morale, make their tactical fluidity full, remove injuries and bans. You can also change a players whole identity, from their nationality, to their number of international caps that they have had and position they play. Name, age, the language they speak, their favoured personnel and favourite/disliked teams. Their wages and how long their loan deal is. A personal favourite of mine is the "Become unsackable" option for yourself, as well as the ability to change your own profile, too. Attributes, age, favoured and disliked teams and personnel, nationality and more.
In comparison to the main editor, you can't swap teams in different leagues, create, bring back from and retire players. I think that's about it, really.