Q:

What do these numbers and symbols on the ICAO SIGMET chart represent?

Anybody who can identify the numbers and alphabets in the red, green, and purple circles on the chart above?

A:

Green are latitude and longitude.

The purple are major cities, e.g. A for Accra and B for Brazzaville (west coast of Africa traced in red above).
For the top line, here's an example breakdown:

PGCE05 represents the reference to the specific chart.
EGRR represents the WAFC issuing the data (in this case WAFC London. KKCI represents WAFC Washington). In the case of a backup situation, this will remain the same even if the data is issued by the other WAFC.
161200 represents the day of the month (16 in this case) and the hour in UTC (1200 in this case) of the analysis on which the forecast is based. Note, this time will always be 24 hours earlier than the validity time of the forecast as specified in the main legend on the PNG forecasts.

Source: Guidelines for interpreting World Area Forecast Centre Significant Weather forecasts

