Main markets consist of order sizes that meet the board lot or the standard lot size number. Most trades usually occur in the main market; therefore, there is more liquidity in this market.
On the other hand, any order size that falls below the board lot is classified under the odd lot market.
Please note that each market has its own order book (list of buy and sell orders); therefore, if you are looking at the Market Depth widget of a certain security, make sure to switch to the right market by picking either MAIN or ODD in the drop down list.