I don’t understand the founder effect?

I think that the founder effect is where members of the original population migrate and establish a new sub-population in a different area.
As the number of individuals in the new sub-population is v small (lets say 100) its gene pool will not be as diverse as the original population (with a population of lets say 1,000,000)

The genetic bottleneck, on the other hand, is where the original population size shrinks - for example due to natural diasters or deforestation. This means that a fewer number of individuals survive in the population - the gene pool is reduced. The population then grows again, from these surviving individuals

I think the underlying concept is the same - genetic drift, except founder effect is due to migration and the genetic bottleneck is due to a disruptive change in the habitat.

here's another way to word it :

The founder effect is when specific alleles of genes may be lost from a population when some organisms become isolated from the main population and "found" a new separate population. This new population contains a random selection of alleles from the parent population

The bottleneck effect is when the majority of a population may die out due to natural disaster such as tsunami, flood or fire and a large number of genes are lost. However, survivors carry a small random selection of genes from the original population. This results in a change in frequency of alleles in a population and significant loss of genetic variation.