Psalms - Chapter 110
You May Want to Have Genesis 14 Handy
According to Rashi (who quotes earlier sources), David dedicated this Psalm to Abraham, who was the first to call G-d "my Master." (Of course, we emulate this form of address every time we speak to or of G-d in prayer, as the Name by which we call Him means "my Master.")
G-d told Abraham to stand by (on His right) while He, G-d, defeated his enemies. G-d promised to send bread from Jerusalem; this refers to provisions sent by Malki-Tzedek, king of Jerusalem in Abraham's day, following the war of the kings (see Genesis 14:18). G-d will rule among the enemies of the nation, referring to the ultimate conquest of Jerusalem (in the time of David) and its establishment as the capital.
The members of Abraham's household voluntarily fought alongside him when he battled the kings (see Genesis 14:14-15). They did this because Abraham was a uniquely pious individual, who recognized G-d when he was but a very small child. Abraham was renowned for his acts of kindness, which are compared here to a gentle dew.
G-d has promised Abraham that kings and priests would be among his descendants and He will keep His word to do this, in accordance with Malki-Tzedek's blessing to Abraham (Genesis 14:19).
G-d is at Abraham's right, conquering the four kings (in Genesis 14, again). In Abraham's future, He will judge Egypt and leave that nation strewn with bodies. The descendants of Abraham will triumph over the mighty nation of the Nile, enabling them to proudly lift their heads. (These last two verses are the conclusion of the Av HaRachamim prayer recited after the Torah reading on most Sabbaths.)
You will notice that, at the beginning of the Psalm, G-d placed Abraham to His right. At the end, G-d placed Abraham to His left (with the result that G-d was on Abraham's right). Of course, this whole description is metaphorical, as G-d has no physical form and, therefore, neither right side nor left.