Starting in glibc 2.27, a new heap feature called the tcache was released. The tcache was designed to be a performance booster, and the operation is very simple: every chunk size (up to size 0x410) has its own tcache bin, which can store up to 7 chunks. When a chunk of a specific size is allocated, the tcache bin is searched first. When it is freed, the chunk is added to the tcache bin; if it is full, it then goes to the standard fastbin/unsortedbin.

The tcache bin acts like a fastbin - it is a singly-linked list of free chunks of a specific size. The handling of the list, using fd pointers, is identical. As you can expect, the attacks on the tcache are also similar to the attacks on fastbins.

Ironically, years of defenses that were implemented into the fastbins - such as the double-free protections - were ignored in the initial implementation of the tcache. This means that using the heap to attack a binary running under glibc 2.27 binary is easier than one running under 2.25!

Tcache poisoning is a fancy name for a double-free in the tcache chunks.