What you want to Google is man bash
There are a number of ways to refer to a job in the shell. The charac-
ter % introduces a job name. Job number n may be referred to as %n. A
job may also be referred to using a prefix of the name used to start
it, or using a substring that appears in its command line. For exam-
ple, %ce refers to a stopped ce job. If a prefix matches more than one
job, bash reports an error. Using %?ce, on the other hand, refers to
any job containing the string ce in its command line. If the substring
matches more than one job, bash reports an error. The symbols %% and
%+ refer to the shell's notion of the current job, which is the last
job stopped while it was in the foreground or started in the back-
ground. The previous job may be referenced using %-. When there is
the current job only, %- refers to the shell's notion of the current
job. In output pertaining to jobs (e.g., the output of the jobs com-
mand), the current job is always flagged with a +, and the previous job
with a -. A single % (with no accompanying job specification) also
refers to the current job.
Simply naming a job can be used to bring it into the foreground: %1 is
a synonym for ''fg %1'', bringing job 1 from the background into the
foreground. Similarly, ''%1 &'' resumes job 1 in the background,
equivalent to ''bg %1''.
TL;DR: %1 is job number 1.
