Solid flooring has a tendency to expand and contract. In summer, when the air is more humid due to the house being aired more often - the boards soak up the extra moisture and tend to expand slightly. In winter, when central heating / underfloor heating are used regularly, the air in the house is very dry therefore resulting in the boards drying and contracting - this often creates gaps appearing between boards - often large enough to put a coin between. These are harmless and disappear in the warmer months when heating is not used in the house as often. https://homespure.com/best-carpet-for-home-office/