An independent-samples t-test compares the means between two unrelated groups, such as comparing the difference between class 1 and class 2, another very common example would be used to compare a binary gender difference.
If you have more than three groups consider using a One-Way Between-Subjects ANOVA
It is considered a parametric test and is only suitable for parametric data. To check if your data is parametric, please read our dedicated guide: Parametric or Not Guide (PDF)
If your data is non-parametric you should consider using a Mann-Whitney U-Test.
SPSS will generate three tables, for this test we need to read from: "Group Statistics" and "Independent Samples Test".
This table shows a selection of descriptive statistics: the sample size of each group (N), the mean of each group (Mean), and the standard deviation of each group (Std. Deviation), these are typically all reported.
This table shows the specific test results including the t-statistic (t), the degrees of freedom (df) the two-tailed significance or p-value (Two-Sided p), and the 95% Confidence Interval (95% Confidence Interval of the Difference).