When girls join Brownies, there is a period of at least 4 meetings before they can make their Promise and become Brownies. As such, some units opted to have a special name for them. The challenges to be done before making the Promise were known as the Tenderfoot challenges - drawn from cultures where hunting was done barefoot, and those new to joining in hunting literally had tender feet until they became hardened with regular hiking over country. In some units, they saw the girls as being between ordinary and Brownie, so they referred to them as 'Tweenies', to describe their in-between state.
Nowadays, when there can be far longer periods between a girl joining the unit, and feeling ready to take on the commitment involved in making the Promise, these names are less common, and girls usually perceived as being full-Brownies from the start.