“Just the tip” is a slang term that refers to just the tip of the penis, and may be used as a way to get a partner to agree to penetrative sex. The tip of the penis likely refers to the head or glans. It is the most sensitive part, but makes up a small part of the penis overall.
The origins of "just the tip" are not clear, although it did appear in a very famous scene in the 2005 movie "Wedding Crashers." The first reference to it in Urban Dictionary also appeared in 2005.
For people who are new to penetrative sex or aren't sure they'd like to have it, "just the tip," may be used as a way to convince a partner to allow penetration, whether anally or vaginally. The phrase is often used tongue-in-cheek, however, because it's unlikely that the penetrating partner will be able to resist thrusting, as this tends to be what makes sex feel good.