СССР Parts Identification
From this page (in Russian), we can find that:


МБГО stands for конденсатор металлобумажный герметизированный однослойный, i.e. metal-paper hermetically sealed single-layer capacitor
МБГО-1 differ from МБГО-2 by means of mounting: the latter ones have mounting elements on the case


1 мкФ is the Russian equivalent for 1 μF, while 300В means 300V. ±10% means that actual capacitance is within 10% of the nominal value.

If you try to search for description of Soviet parts on the Internet, you may want to look for Russian-language pages with the keyword of "маркировка", which means "marking", and use Google Translate to translate them into your language.