In the modern musical system, A440 or Concert A is the note which has frequency of 440 Hz and serves as the standard pitch from which the frequencies for all the other notes can be calculated.
The system where an octave, divided into 12 notes (12-TET), has its A note = 440 Hz is thus referred to as standard tuning.
For a guitar, standard tuning has a slightly different additional meaning. It means that the 6 stings are tuned from above downwards to the notes as follows -
6th string = E note (Thickest string) - 82.4 Hz
5th string = A note - 110.0 Hz (= 440/4)
4th string = D note - 146.8 Hz
3rd string = G note - 196.0 Hz
2nd string = B note - 246.9 Hz
1st string = E("e") note (thinnest) - 329.6 Hz
The A note (5th string) here, is not = 440 Hz. Instead of that, it uses a lower harmonic = 110Hz. The strings would not be playable, if this A were 440Hz. As sounds that are harmonics sound just the same (just softer) and have similar properties, the system allows the guitar to be in tune with higher strung instruments.
The basic idea here is to tune the instrument in ascending perfect fourths (E to G and B to e) while B to G are major thirds.
Also read: Non-standard tuning systems
Liked 'Standard tuning : What is it?' enough to share / save?