The "payroll" refers to MPs who hold roles in the government, i.e. government ministers, parliamentary private secretaries (PPS), whips or in some way dependent on the PM's patronage. These people may be at risk of losing their jobs in the government if there is a change of leader.
From the House of Commons website:
There is no formal definition of the payroll vote. It is generally considered to refer to all those who hold a role in the administration, whether paid or unpaid. This includes senior roles, as well as more junior roles including parliamentary private secretaries (PPSs). The proportion of Members of the House of Commons who have been part of the payroll vote varied from 19-22% between 1979 and 2017.
Not being on the "payroll" simply means one is a backbencher MP.
The breakdown of the payroll vote is as follows, according to the Institute for Government. Keep in mind that not all on the payroll are paid. PPS and a small number of ministers are unpaid.
The current payroll vote is between 160 and 170 MPs, consisting of:
95 ministers (including whips) in the House of Commons
47 parliamentary private secretaries (assuming all those who also serve on the Privileges Committee have resigned their PPS role to be able to investigate the prime minister)
20 Conservative MP trade envoys
an unknown number of party vice-chairs.
PA News (via The Guardian) explains the significance of the payroll vote in this vote of confidence.
As many as four in five of the MPs who backed Boris Johnson in the confidence vote may have been on the so-called government “payroll”.
Between 160 and 170 MPs currently hold government roles, such as ministers and parliamentary private secretaries, according to analysis by the Institute for Government.
It would be hoped by Downing Street that all of these MPs would have backed Johnson in the confidence vote.
Were this the case, around 80% of the 211 MPs who voted for the prime minister in Monday’s ballot could be said to have done so chiefly out of duty rather than loyalty.
The rest of the 211 MPs who said they had confidence in Johnson will have been backbench MPs who are not on the payroll.
But with as many as 170 payroll votes supporting the prime minister, the figures suggest only a few dozen non-payroll votes also voted in favour, implying that Johnson has lost the confidence of the majority of the Conservative backbenchers.
Thanks to @SteveMelnikoff for pointing out that the 116 ministers previously cited includes those sitting in the House of Lords. This answer has been updated to better reflect the breakdown of the payroll vote.