。
Mother is making a cake.
Literal
Mother [topic-は] cake [obj-を] is-making.
With an activity verb like 作る, ~ている encodes a progressive action in the English sense: 'is making [right now].' Contrast with change-of-state verbs (結婚している = 'is married,' the resulting state, not 'is marrying') — the same form, different aspectual reading.