The LNAT is an online aptitude test used by certain universities to help them to shortlist candidates. It tests general comprehension and reasoning skills (not legal knowledge) via a combination of multiple-choice questions (which make up your LNAT score) plus an essay. You can take it before or after submitting your UCAS form but can only take it once per admissions cycle. The LNAT Universities in the UK are Oxford, LSE, UCL, Kings, Durham, Bristol, Nottingham, Glasgow and SOAS (not compulsory at SOAS for A level/IB students although a high LNAT score may result in a better offer).
The average score varies from year to year and there is no official pass mark. However, Nottingham do have a cut off point below which people will be rejected or offered alternative courses, which in 2019 was 21. In the 2019/20 cycle, both Nottingham and Kings seemed to give out a lot of alternative offers to people with good A levels/IB but who got an LNAT score of 20 or below.
As a guide, the average Multiple choice score for successful applicants in 2019/2020 was 21.5 and 23 in the previous cycle. However, many successful applicants scored lower than this while successful applicants to places like Oxford & UCL tended to score higher. Only Oxford, UCL and Bristol give the essay a numerical score, although Kings uses it to distinguish between borderline applicants and Durham considers it "holistically" along with other aspects of the application. LSE do not currently consider the essay at all in their assessment of applicants.
In order to guarantee that your application will be considered, you need to sit the LNAT by 20th October 2019 for Oxford, or 20th January 2020 for all the other institutions. If applying to Kings, they encourage their applicants to sit the LNAT by mid December.