The four metrics in Search Console's Performance report (clicks, impressions, CTR, average position), the views you can switch between, and cautions when reading it, per Google's help.
The four metrics
According to Google's official help, the report shows the following metrics. Clicks are the number of times users went from a search result to your site. Impressions are the number of times your site appeared in search results.
CTR is clicks divided by impressions. Average position indicates the average rank of the topmost result from your site. The chart is calculated for the whole site, while the table is calculated per row, such as a URL or a group.
Switching the view
The table can be switched between dimensions such as queries, pages, countries, devices, search appearance and dates. The default date range is the last three months, and you can change it with the date filter. The 24-hour view shows data by the hour.
The search type filter lets you look at web, image, video and news results separately. For long-term trends, a weekly or monthly view smooths out daily ups and downs and makes the overall direction easier to see.
What to keep in mind
Google's help notes that search results vary with the searcher's time, location, device and history, so searching for a query listed in the table yourself does not necessarily show the same results.
The chart and the table also use different units of aggregation, so their totals may not match. The most recent data can change within a few hours and is shown with a dotted line. When you export to CSV or similar, values shown as '~' or '-' are output as zero.
This article is based on the official documentation, and the specifications may change.
TAKEAWAY
What this means for your project
Switch to the monthly view over the default three months and check whether impressions and clicks are rising or falling. Then look at the queries table for your brand name and key service terms, and the pages table to find pages that changed. Use those pages as a starting point for a content review or a question to your vendor, rather than reacting to daily swings.