percent

Bring most recent background job to foreground

$ %

$ %[N]

$ %[string]

$ %?[string]

$ echo $((10 % 3))

% [jobspec_]

% in shell context typically refers to job control, bringing background jobs to the foreground. It's shorthand for the fg command.
When a command is suspended (Ctrl+Z) or run in background (&), it gets a job number. The % notation provides quick access to these jobs.
In arithmetic contexts $(( )), % is the modulo operator, returning the remainder of integer division.
In prompt strings (PS1), % introduces escape sequences in zsh for dynamic prompt elements.
In printf/strftime, % introduces format specifiers.

% or %+ or %%

Current (most recent) job

Previous job

Job number n

Job whose command starts with string

Job whose command contains string

Job control requires an interactive shell. Not available in scripts by default.
The % interpretation is context-dependent: job control at command position, modulo in arithmetic, format specifier in strings.
Job numbers reset when the shell restarts.

fg(1), bg(1), jobs(1), kill(1)