This page of options constrains your search on the casting elements of a card.
Each color component (black, blue, green, red, white, colorless) has three fields. First, check the boxes next to the colors you want to search on. Note that you can also constrain the colorless mana component
Each color component also has a dropdown list to select how you want to constrain the color component. 'in casting cost' means that the card has that color in it's casting cost. The other settings require you to specify a value as well for the comparison. The value is selected from the dropdown list to the right of the constraint list.
You can change the 'Search type' as well. 'match all colors' means that each matching card must meet all of the color constraints specified. 'match any color' means that cards matching any one or more of the color constraints is sufficient.
'X' in casting cost lets you search only for cards with variable elements as part of the casting cost. Note that this does not find cards with cariable costs that are described within the card text, rather than the casting cost line.
Total casting cost lets you search on the total casting cost of the card, regardless of the colors. Note that this assigns zero to any variable 'X' components in the casting cost of cards.