The "third world" were originally the countries which were neither capitalist -nor communist - or not aligned with the US or Soviet Union. Both US and the Soviet Union tried to "win them over" - to increase their sphere of influence and eventually to have a bigger power.
Science forums were one of the many ways of inviting the third-world countries into a closer contact with the Soviet Union (or the US).
Soviet Union for example invited scientists to their universities and offered monetary compensation for working with them