The term matters because public Torah reading in Judaism follows a structured cycle.
A sidra is the weekly Torah portion
Britannica defines sidra as the weekly reading from the Scriptures as part of the Sabbath service. Each week a portion of the Pentateuch is read aloud in synagogue, and over the course of a year the cycle is completed.
That is the basic meaning Jews usually have in mind when they talk about the weekly portion.
The cycle is a communal discipline
Britannica notes that public Torah reading was extended to Sabbath services in order to make the laws of Jewish life accessible to all.
That historical point matters. The sidra is not simply a reading schedule. It is one of the ways Jewish communities keep Torah publicly present.
The weekly portion is subdivided
Britannica also explains that each sidra is divided into seven parashot, which correspond to the multiple aliyot in synagogue reading.
This is why terms like sidra and parashah overlap in casual speech but are not identical.
Why it still matters
The sidra still matters because it gives Jewish study and worship a shared weekly rhythm. Around the world, communities return to the same Torah portion together.
The shortest accurate answer
A sidra is the weekly Torah portion read in synagogue as part of the annual cycle of public Torah reading.