Short answer: they're making a right angle with the Earth, so they appear 90 degrees apart on the apparent sphere of the night sky if we use the center of the sphere (our subjective position viewing the sky) as the third point. Or to put it another way, if you stretched one arm to point at Mars and the other to point at Saturn, your two arms would form a 90 degree angle with your body as the center point.
Long answer: because the planets all orbit in roughly the same plane, and the Sun is also in that plane, from our perspective their movements are restricted to a thin circular band in the "sphere" of the night sky (the celestial sphere), a band that crosses through the 12 constellations that make up the Zodiac. Here's an image from the wiki Zodiac article illustrating why we see the Sun move in a circle relative to the celestial sphere (the small red circle is our orbit around the Sun, the yellow line is our line-of-sight viewing the Sun relative to the stars, and big red circle the is apparent path of the Sun relative to the stars over one year):
And an image from this page shows how the plane of the Earth's orbit intersects the celestial sphere in a circle that crosses the constellations we have deemed to be "the Zodiac" (and this is also close to the plane of the other planets' orbits, so the line-of-sight between us and another planet should intersect the celestial sphere close to this circle, and the line-of-sight between us and the Sun intersects exactly on the circle, which is known in astronomy as the ecliptic):
So for this reason, in astrology the positions of the planets on a certain date are depicted by the placement of their planetary symbols on circle on a natal chart, with the circle being divided into 12 "houses" that originally corresponded to the position of the 12 constellations of the Zodiac (though they don't any more due to a long-term change in the direction of the Earth's access called precession):
The image comes from this page which explains roughly how it's supposed to work, for example the red and blue lines connecting different planets are called "aspects". And according to wikipedia's astrological aspect page:
In astrology, an aspect is an angle the planets make to each other in the horoscope, and also to the ascendant, midheaven, descendant, lower midheaven, and other points of astrological interest. Aspects are measured by the angular distance in degrees and minutes of ecliptic longitude between two points, as viewed from Earth.
So, presumably a 90 degree angle would mean they are 90 degrees apart on the circle, with the center of the circle (corresponding to our position on Earth looking up at the circle in the night sky) as the middle point that the angle is measured relative to.