Without any options, pgrep <name> only returns the PIDs of processes matching the name <name>. For example:
$ pgrep vim
23084
23085
A more useful combination of flags is -fl:
-f: matches against the full argument list, not just the process name.
-l: long output, also prints the process name. When combined with -f prints the whole argument list.
Some examples:
$ pgrep -l vim
23084 nvim
23085 nvim
$ pgrep -fl vim
23084 lvim -u /Users/diegoallen/.local/share/lunarvim/lvim/init.lua main.go
23085 lvim --embed -u /Users/diegoallen/.local/share/lunarvim/lvim/init.lua main.go