First sorry for my bad English,
well… I make a video for explane better what is the problem so… here it is: https://www.youtube.com/watch?v=a8io8H-dYto
(I Alredy Seach for Forums and Tutorials but I don’t find anything)
@MaryCat_123 Hi. First of all welcome to the forum.
The “onion skin” button on the timeline, is a misnomer (i.e it’s not named properly). It’s actually used to switch the “match keyframe” mode with lets you see onion skinned adjacent frames or drawings (keyframes)
To learn how to enable the onion skin please look at the FAQ forum thread here:
Thx! ^^ and sorry for make a question that is already answer 
, I will Search more next time!