Space is often illustrated by 3 lines representing the three spacial dimensions. They are the axis of a 3d graph. In 3d space you can move in 3 directions forwards and backwards; up and down and left to right. General Relativity depicts space and time as a fabric that can be warped, bent, twisted, stretched and contracted. Quantum Mechanics describes subatomic particles coming and going even in a vacuum. Space is not empty, it is not nothing. Yes I know this is a double negative however empty space is normally thought of as nothing but it is something.
Basic SI unit – meter (m) which is the distance traveled by light in a vacuum in 1/299,792,458 of a second.
The most common methods of measuring space are rulers but odometers are another common way of measuring distance and there by space. Other methods include bouncing light, radio waves, or sound off of an object using the return time to measure distance. These include radar and sonar. Parallax uses geometry to measure distance it is used in Astronomy and land surveying.
Distance and position must be measured according to some standard. The standard has to be a chosen point in space. The point is selected by ether convenience or convention with out such a starting point determining distance and position is impossible..
Space is one of the three main parts of the Universe space, time and mass. Relativity shows them to be inter-related with space and time are so interrelated that they form a single entity. Intern mass warps space-time forming gravity and motion of mass contracts both space and time.
In conclusion based on common experience space is basically where you are. You are at a particular point in space relative to other objects. Physics however has shown it to be a lot more. It is a fundamental part of the universe.