Protoevangelium is a compound word.  Proto means first.  A prototype is the first creation of some item.

Evangelium sounds a lot like the word evangelize, and it is.  When you evangelize, you share the gospel.  That's what evangelium is.  The gospel.

So, protoevangelium is the first gospel.  It is the first good news.  It is found in Genesis 3:15 when God first tells us that he is going to send Jesus (even though he doesn't mention Jesus by name!)

After the Fall of Man, God pronounces judgment on man for his sin.  But He also offers the good news that the seed of the woman, Jesus, would crush the head of the serpent, Satan.  Satan would strike Jesus on the heel, his death on the cross, but it would not be mortal as Jesus would rise from the dead.  Jesus would strike back with a mortal wound to Satan by crushing his head.  The protoevangelium is God's first announcement that Jesus was coming and that Satan would be defeated.