[R] What are Type II or III contrast? (contrast() in contrast package)

[Peng Yu]

?contrast in the contrast package gives me the following description.
However, I have no idea what Type II and III contrasts are. Could
somebody explain it to me? And what does 'type' mean here?

    *‘type’*: set ‘type="average"’ to average the individual contrasts
    (e.g., to obtain a Type II or III contrast)