Why isn't the whole root partition being used? [duplicate]

I have a machine running Ubuntu 22.04. It has a 128GB hard drive. The root partition is roughly the whole thing, and I confirmed this with parted (it's #3 in the list):

Model: ATA TS128GSSD370S (scsi)Disk /dev/sda: 128GBSector size (logical/physical): 512B/512BPartition Table: gptDisk Flags: Number  Start   End     Size    File system  Name  Flags 1      1049kB  1128MB  1127MB  fat32              boot, esp 2      1128MB  3276MB  2147MB  ext4 3      3276MB  128GB   125GB

I also confirmed with gparted, which shows the filesystem type as "lvm2 pv":

However, when I run df, it shows the partition is only 57GB and is 94% used:

# df -hFilesystem                         Size  Used Avail Use% Mounted ontmpfs                              369M  2.1M  367M   1% /run/dev/mapper/ubuntu--vg-ubuntu--lv   57G   51G  3.8G  94% /tmpfs                              1.8G     0  1.8G   0% /dev/shmtmpfs                              5.0M  4.0K  5.0M   1% /run/lock/dev/sda2                          2.0G  429M  1.4G  24% /boot/dev/sda1                          1.1G  6.1M  1.1G   1% /boot/efitmpfs                              369M  1.7M  367M   1% /run/user/1000

My expectation is that df would show the partition to be the full 125GB.

Why does the system think the partition is only 57GB when it's really 125GB? Will I run out of space when I hit 57GB of data and, if so, how do I make it use the whole partition?