An aptitude stage tests general reasoning rather than knowledge of a particular technology: numerical, verbal, logical and situational judgement, depending on what the role asks for.
How it differs from an MCQ assessment
An MCQ assessment tests what you know. An aptitude test tests how you think, and you cannot revise for it in the same way. Both are timed, both are multiple choice, and both run in the same player.
Timing
Aptitude sections are usually tightly timed on purpose — the pace is part of what is being measured. Answer the ones you can, and move on rather than stalling. Where the test does not penalise wrong answers, an educated guess beats a blank.
Sections
A test can have several sections, each with its own type and its own clock. You cannot usually return to a section once you have left it; the player says so before you move on.
What is recorded
Your answers and your timing. If the organisation enabled monitoring for this stage, the same camera, microphone and screen rules apply as for any other stage — you will be told before it starts and asked to agree.