Implicit is implied, rather than directly stated. A facial expression can be an implicit signal. But, any hint you give indirectly is an implicit signal.
I'll try to give some examples:
Let's say you have 2 sauces on a table, a green one and a red one, and I'm trying to tell you that the red one is spicy.
"Do not eat the red sauce! It's way too spicy." - Very explicit
"I think the green sauce is better, I don't like spicy things that much" - Implicit. I'm saying I don't like the red that much, and I don't like spicy things. Since the two pieces of information are given at the same time, you can probably guess the red one is spicy.
"Wow, a little bit of the red sauce is more than enough." (while I drink water) - Implicit. So we know the red sauce must be strong. Since water calms spiciness, and I'm drinking water, it's likely that the red sauce is spicy.
Credits to my friend eva!