Coverage and read depth are metrics that look at how well your library has been sequenced. Visually you can picture coverage looking at it horizontally, while read depth looks at it vertically.

Coverage, or coverage breadth, answers the question “how much of the sample is covered by sequencing?”. It can be reported as the percentage of the bases covered by sequencing reads, e.g. 95% coverage indicates that 95% of the bases in the sample have been sequenced (at depth n). 

Read depth, or sequencing depth, instead indicates how many reads detected a specific nucleotide. This can be a strong indicator of the reliability of a base call. Low read depth can also indicate that a specific region is poorly represented in the sample. Generally, you can say that if region/base call has 30 reads mapping onto it, then it has a 30x read depth. Read depth can also be indicated as average read depth, which is calculated taking in consideration the total number of bases sequenced and their individual depth. Generally, the deeper the sequencing, the more reads you have.