There is significant debate over how John 1:15 should be interpreted (see the question Does John the Baptist's witness imply the pre-incarnate existence of Jesus?).
"John testified concerning Him. He cried out, saying, “This is He of whom I said, ‘He who comes after me has surpassed me because He was before me.’ ”"
An important point in favour of the temporal interpretation of 'before' is Jesus' statement at John 8:58, which is also a point of significant controversy in terms of proper translation, but is something like
"“Truly, truly, I tell you,” Jesus declared, “before Abraham was born, I am!”"
If we assume that John 1:15's 'before' is to be understood temporally, and we note that 1:15 says John is testifying concerning Jesus, it seems reasonable to hold the phrase "He was before me" would have meant something important to John the Baptist's audience (otherwise it wouldn't have meant much as testimony).
Note this is reiterated at John 1:30, but there John the Baptist explicitly claims a man comes after him who was before him.
Similarly, at 8:58, when Jesus says He was before Abraham, the audience seems to understand this as having a certain kind of meaning - it means something important.
Yet note at 1:15 no one is picking up stones to kill John the Baptist for claiming this about another man, while they do pick up stones at 8:58 when Jesus says something similar (on our assumption 1:15 is meant temporally).
So, what are our options for a plausible interpretation of how John the Baptist's claim that Jesus was temporally before John the Baptist would be understood by devout 1st century Jews? In particular, which figure or type of being would they have thought he might be indicating by saying this someone was temporally before him?