天 (tian) means 'sky', 'the heaven‘ or ’the space'.  You may think that it is '大 '（da) plus '一'（yi).     ’大‘ means ‘big' and ’一‘ means ’one' or ‘first’.  Everyone should admit that ’the sky is the first biggest thing in the world'.  The sky is the roof of the Earth and the former can cover every thing on the Earth. 

About 地（di)：Its first part is a simple version of 土（tu) that refers to ‘earth';  也 is a special word usually at the end of a sentence in ancient Chinese for making a judgement.  So 地 means ’ground‘, 'land‘，’field‘，’earth‘ or ’the Earth'.

The whole phrase ‘天地’ may mean ‘the Universe'.