low-level versus high-level
Picture two ways to give directions. High-level: 'go to the library.' Low-level: 'turn left, walk 200 metres, cross at the lights, turn right...' Both get you there. The high-level version trusts you to fill in the steps; the low-level version spells out every move. Programming languages and code sit on the same scale, and 'level' simply measures how close the instructions are to the machine's own tiny steps versus a human's larger intentions.
A high-level language (Python, JavaScript, Java) lets you say what you want in terms close to the problem - sort this list, fetch this URL - and a lot of machinery underneath turns that into actual machine steps. A low-level language (C, and below it assembly) makes you express things much closer to what the CPU and memory actually do - move this value into that location, add these registers. Lower level means more control and detail and usually less hand-holding; higher level means more convenience and safety and usually less direct control over the machine.
Why this framing matters: it is the single most useful axis for placing a tool or a piece of code. It predicts the trade-offs you will face - speed and control versus convenience and safety - and it explains why systems programming lives toward the low-level end. Be careful, though: 'low-level' is not an insult and 'high-level' is not lazy. They are different jobs. The skill is picking the right level for the task, and knowing enough about the low level to understand what the high level is really doing.
To add two numbers, high-level C lets you write c = a + b. The equivalent low-level assembly might be three lines: load a into a register, add b to it, store the result. Same effect; the lower level shows every machine step.
One high-level line; several low-level steps that the CPU actually runs.
The terms are relative and have drifted over time: C was called 'high-level' in the 1970s next to assembly, yet today it is the textbook 'low-level' language next to Python. Always ask 'low/high compared to what?'