Singer and Miku user here! I'll try to explain it as simple as possible: accent is the emphasis of the consinant part of a word to make it more powerful and emotional. In shedt music, some words can be marked with accent marks as guideline to help a singer. Really, accent is the power or softness of the consonant to help an entrance of a word, usually going along with dunamics. If you start lohd and agressive, you'll want lots of accent to be rong, if you want to be soft and sweet, have a soft accent as to not overpower the vowel's tome.
This holds true in EVEC too. Think of it as an easier way to put emphasis than changing the velocity of the note. Keep in mind, however, accent usually occurs only at the start of a phrase or at the end of one. It doesn't really matter given it's able to be used regardless of placement in terms of Vocaloid.
Thevothers explained the Vocaloid side of accent in EVEC so I thought I'd focus on the actual singing side more. I think it's more helpful to know the actual singing technicality and use before the Vocaloid use, but that's just me