I heard that; it is said that
Expresses hearsay or reported information, meaning 'I heard that' or 'it is said that'.
This そうだ is used when you are reporting something you heard from someone else. It shows that the information is not from your own direct experience. For example, if a friend told you that a new restaurant is good, you might say 'あのレストランはおいしいそうだ' (I heard that restaurant is good). You are not saying you tried it yourself, just relaying what you heard.
Formally, it can also be used for general knowledge, like '日本は四季があるそうだ' (It is said that Japan has four seasons). In speech, it can be shortened to そう, as in '明日は雨が降るそう' (I heard it will rain tomorrow), but this is casual.
Be careful: there is another そうだ that means 'looks like' (e.g., おいしそうだ = looks delicious). That one attaches to the stem of verbs and adjectives (e.g., 食べそうだ, 楽しそうだ), not the plain form. So 'おいしいそうだ' (hearsay) vs 'おいしそうだ' (appearance) are different.