The y-intercept is the point where the best-fit line meets the y-axis, which is the value of y when x equals zero. In physics it often carries meaning, such as a starting value or a constant offset in a relationship.
The most important rule is that you can only read the intercept directly if the x-axis begins at zero. If your scale starts at some other value, the visible crossing on the left edge is not the true intercept, and reading it there is a common error.
When the axis does start at zero, extend your best-fit line if necessary until it reaches the y-axis, then read off the y-value at that crossing using the scale.
If the line clearly passes through the origin, the intercept is zero, which itself can be a meaningful result showing direct proportion between the two quantities.
What examiners award
Marks are awarded for reading the y-value where the line crosses the y-axis at x equals zero, correctly using the scale, when the x-axis starts at zero.
Common mark losses
In short
The y-intercept is the value of y where the best-fit line crosses the y-axis at x equals zero. Read it only when the x-axis actually starts at zero.
A routine that works
Practise the mechanics until they are automatic: label both axes with units, choose a scale that fills more than half the grid, plot points with a sharp pencil, draw a single best-fit line, and take the gradient from a large triangle with its coordinates and unit shown. One clean example beats re-reading the theory.
Source: DSKP KSSM Physics Form 4 and 5 (Versi English) (Bahagian Pembangunan Kurikulum (BPK), KPM)