Sodium is a mineral you mainly get from table salt. In your body, sodium binds water. That is why salt is considered one of the reasons your weight fluctuates from day to day without any change in body fat.
The effect on the scale
A salty dinner, restaurant food, fast food, ready meals or a bag of chips can shift your water balance. The effect sets in within hours and fades on its own over one to three days, once your kidneys have excreted the excess. The other way round, after a day with little salt and lots of movement you may look "better" on the scale.
What the research says
The evidence is less clear-cut than everyday experience suggests. In a controlled study with 32 men (Heer et al. 2000), a sustained high salt intake raised blood plasma volume but neither total body water nor body weight. That study looked at a state after adaptation, though, not the sudden salt spike of a single day.
To put it honestly: salt probably contributes to a jump after a restaurant night, but by how much cannot be quantified. Glycogen and gut content are the better documented factors, and often all three come together.
What to do
Nothing special. Log the reading after a salty evening like any other and only read your trend: a well-smoothed trend absorbs single outliers. Do not react by eating less or trying to "drop water"; that only changes water. Adjust calories only when your trend deviates from your goal over two to three weeks. A short note on the day helps you recognize such spikes later.