What does TRIM on an HDD mean?

This probably means that your HDD is an SMR drive. Since SMR drives work differently, a TRIM operation is needed to improve performance over time.

WD's support page provides a good explanation for this:

TRIM/UNMAP is supported for external hard drives with SMR (Shingled Magnetic Recording) drive inside, like SSDs, for dataset management and to improve SMR performance over time. One of the shingled write benefits is that all physical sectors are written sequentially in a direction radially and are only rewritten after a wrap-around. Rewriting a previously written LBA (Logical Block Addressing) will cause the previous write to be marked invalid and the LBA will be written to the next sequential physical sector. The TRIM/UNMAP enables the OS to inform the drive which blocks are no longer considered to be in use and can be reclaimed internally by the HDD to ensure that later write operations perform at full speed.

The documentation for this drive from Seagate lists the SATA commands it supports, and does not include TRIM in that list.

Said documentation based on the make and model reported by the displayed screenshot:

https://www.seagate.com/files/www-content/product-content/seagate-laptop-fam/barracuda_25/en-us/docs/100818135e.pdf

Today i was suprised when a new ST2000DM008 arrived, and it claims support for TRIM. I noticed this after lvcreate was trimming the free space.

SMART reports the trim support. Kinda hints that both my and yours are SMR drives.