Linkage is the physical association of two genes present on the same chromosome such that they have chances to get inherited together as a unit from parents to offspring.
Strength of linkage is inversely proportional to the distance between the genes.
The closer the genes are located on the same chromosome, the lesser is the chances of crossing over between them. Hence, they are said to be tightly linked.
If the distance between the genes is more, then the genes are loosely linked and there is a greater chance of crossing over.