To mark one or more positions in a file, use the m[ark] command.

Examples:

ma       -    set current cursor location as mark a

'a       -    jump to beginning of line of mark a

`a       -    jump to postition of mark a

d'a      -    delete from current line to line of mark a

d`a      -    delete from current cursor position to mark a

c'a      -    change text from current line to line of mark a

y`a      -    yank text to unnamed buffer from cursor to mark a

:marks   -    list all the current marks

NB: Lowercase marks (a-z) are valid within one file. Uppercase marks
(A-Z), also called file marks, are valid between files.

For a detailed description of the m[ark] command refer to

:help mark

See also:

:help various-motions

from here.