What is a Beat ?

A Beat is the basic foundation of time in music.
Tempo ( speed ) of music is defined as Beats Per Minute ( bpm ) - This ranges from 40 bpm ( very slow ) to 200 bpm ( very fast ).

Each tick of a metronome is a beat
This underlying beat is what you could tap your foot to in most ( but not all ) forms of music.
Do not confuse this definition of a beat with drum patterns which can also be called beats.
Also do not confuse beats with hits, strokes, strums or picks. For instance you could have eight or more hits, strokes, strums or picks in a bar which has only 4 beats

Some other uses of the term "beat"

The Down Beat - refers to the first beat in a bar - this comes from classical music where the conductors baton moves down at this point.

The Up Beat - refers to beat 4 of a 4/4 bar - again it's what the baton is doing at this point.

Back Beat or Backbeat - refers to a rhythm where the accents (e.g. snare drum hits ) are on beats 2 and 4 of a 4/4 bar.
Most rock and pop music uses this style.

Onbeat ( or on the beat ) - refers to playing or accenting the main beats.

Offbeat ( or off the beat ) - refers to notes or accents that fall between the main beats i.e. the "ands" in a simple | 1 & 2 & 3 & 4 & | bar. - Ska and reggae,for example, are heavily accented on the off beat.

Exercise
Tap your foot , clap your hands or nod your head in time with whatever music you're listening to.
Try to discover the underlying "beat" - You could try out a free online tool like BpmChecker where you tap along with a piece of music and it tells you the tempo in beats per minute.