Sak means 'thing' and politik can mean either 'politics' or 'policy', so sakpolitik means… 'policy about things'.
In Swedish, it stands in opposition to idépolitik, which literally means 'policy of ideas', and refers to party ideology and principles. Sakpolitik is a broad category, and can be broken down into policies relating to areas such as schools, healthcare, law and order, foreign policy, and so on.
The key thing is that all these issues are current and require action by politicians (such as introducing legislation, upping funding in a budget, and so on). Translated into English, you might define it as 'policies of substance', 'concrete policies' or 'sector policies'.
Sometimes you'll get what are called intressepartier (interest parties), which only concern themselves with one or more of these concrete, sakpolitiska (that's the plural adjectival form) issues – for example, parties set up to defend pensioners' rights or to oppose a specific project.
So why is this word relevant today? Well, if politicians can agree on sakpolitik, such as the best strategy to improve healthcare or to tackle climate change, they can reach deals despite even large differences in idépolitik.
Currently, no group in Sweden's parliament has anywhere close to a majority, and so cross-party talks are taking place. The leader of the Centre Party, Annie Lööf, has said she wants to discuss sakpolitik with the other party leaders, in order to feel out support from both the left and the right of the political spectrum. If there's enough common ground in this area, Sweden might see some progress in government formation.
Jag vill att vi diskutera sakpolitik, till exempel klimatpolitik
I want us to discuss concrete policies, for example climate policy
Vi måste sätta mer fokus på sakpolitik än ideologi
We want to put more focus on concrete policy than on ideology