If v think whole cosmos as a circle with earth in center of it, whole circumpherence is divided in 12 parts. Because whole circumphrence cast 360 degree on centre, each 12th part will cast 30 degree on centre.
This 30 deg part is called a RASHI.
So Rashi is nothing but a measure/space part in the cosmos.
U being interested in astrology must by now have cramed all Rashi & planets by names & order. These r 1.mesha.2.vrishabh.3.mithun.4.kark.5.sinha.6.kanya.7.tula.8.vrashchik.
9.dhanu.10.makar.11.kumbh.12.meen ( These r ARIES to PICES in english)

http://hats2u.sulekha.com/blog/post/2008/12/let-us-start-with-sun-signs-rashies.htm