The isolation penalty is a multiplier for technology costs, which applies to any isolated country. A country is isolated in this sense if it currectly knows of 18 or fewer other countries. For your own country, you can see how many other countries are known by counting them on the Relationships submode of the diplomacy screen. There are 10 countries on a full page.
The isolation penalty for a country is 0.2 for every country less than 18 which is known. That is, tech costs are multiplied as follows:
#known multiplier 19+ 1.0 18 1.2 17 1.4 ... 14 2.0 ... 9 3.0 ... 4 4.0 3 4.2 2 4.4 1 4.6 0 4.8
Thus, being isolated is a huge penalty to technological progress. The only larger penalty to tech is the draconian whiteman penalty. Getting maps should be the highest priority for an isolated country. See the page How to Get Maps for information on overcoming isolation.
Isolation penalties for all countries are recomputed only upon reloading the game, or when any country declares war. Therefore, even when you get enough maps, you won't necessarily see any change for a while.