The literal meaning of 'broad' is 'wide', as in, for example 'a broad river'. This adjective can also collocate with 'accent' to describe a way of speaking, for example, 'He spoke in a broad West Country accent'. The adjectives 'strong' 'heavy' or 'thick' may also be used in this way. It is also possible to say that someone speaks in a broad dialect. Don't confuse this with the term 'broadly speaking', which means speaking in general terms.