Top = seme
Bottom = uke
Directly translated Seme means "attacker" and uke means "receiver" generally who tops and who bottoms these terms are used to refer to both Yaoi and BL genre when necessary as not all BL has sex scenes or connotations to sex. This does not mean there is no switching some stories have switching where it's called "seke" where it is combination of seme and uke as it is both but there are some cases of "suke" which is a bottom who tops sometimes.
Welcome to the bl world! Here we have several types of genre, but explore it next time, as a bew comer, seme and uke is just the japanese of top and bottom.
Can someone explain me what "seme" and "uke" means?
I know what top and bottom is, but seme? uke?